The present invention relates generally to a heterodyne laser radar (or ladar) receiver incorporating an optical amplifier and polarization optical mixer.
Laser radar systems using the homodyne or heterodyne form of signal detection for improved sensitivity generally utilize a main pulse laser which hits a target and is reflected. The reflected signal is received and is mixed with the laser beam of a frequency shifted local oscillator on a detector.
When the ladar return signal is mixed with the frequency shifted local oscillator (LO), the two signals beat together when incident on an optical detector, creating an intermediate frequency (IF) signal. In the heterodyne mixing process, the local oscillator power is increased until the shot noise dominates the background, detector and electronic noises. This shot noise is inherent in the detection process, and any post detection electronic amplification of the signal amplifies this noise level as well, resulting in no increase in detection sensitivity.
The way to overcome this limitation is to amplify the return signal prior to detection, but an optical amplifier adds noises to the signal. The objective of this invention is to provide a method of rejecting the spontaneous emission noise and allowing the optical amplifiers gain to be realized as a gain in sensitivity.
The following United States patents are of interest.
U.S. Pat. No. 4,240,746--Courtenay et al PA1 U.S. Pat. No. 5,157,451--Taboada et al PA1 U.S. Pat. No. 5,162,861--Tamburino et al
The Courtenay et al patent describes a ladar system which includes a homodyne or heterodyne form of signal detection for improved sensitivity.
The Taboada et al and Tamburino et al patents describe laser imaging and ranging systems which include typical polarization devices.